I believe that on the ''Sucursal Virtual'' (»sucursalvirtual.fibertel.com.ar
) you can change the settings and disable MAC Address filtering, and enabling WPA for some security.
That way you won't have to call Fibertel on the issue.
If you have any questions, I am an user of the service.
Also, about the routing, the best way is to request a non-wireless modem and using that directly, as those don't have any issues with routers, for example I have one of the Scientific Atlanta DPC2100 they use with a (crappy) Linksys WRT54G2, and it was really easy to set up.
Also, if you want a good router over here, I suggest you check out the TP-Link WR941ND on MercadoLibre before going to a place like Compumundo or Garbarino, as you'll end up getting a really bad router, which is over priced.